1. Field of the Invention
This invention relates generally to an electric control element, and more particularly to a control that is responsive to a liquid level indicator and is operative to prevent a fuel tank from inadvertently running dry.
2. Description of the Prior Art
It is known to mount visual liquid level indicators on liquid storage tanks such as storage tanks for fuel oil. The indicator typically includes a float on a linkage mechanically connected to a visible marker element located outside the tank. The level indicator linkage may extend, for example, through an opening in the top of the tank where it is received in a downwardly-oriented transparent bell jar that is threadably engaged to the tank opening, for example with a threaded neck formed at the opening. The bell jar may bear graduation markings in gallons or proportions and the position of the visible element in the jar corresponds to the position of the float in the tank. The indicator provides a readily visible measure of the amount of fuel or other fluid within the tank.
In the case of fuel oil tanks for heaters and the like it is most undesirable to allow the heating oil supply stored in a tank to become exhausted. Running dry frequently occurs at the most inconvenient of times. Furthermore, the oil left in the bottom of a nearly-empty tank typically includes sludge and impurities which have accumulated over time. The sludge and impurities clog filters, flow lines, burners and other heater elements when drawn from the tank into the system. It also may become necessary to bleed out air from the system if the tank supply becomes exhausted. Even when not entirely clogged by sludge and the like, it may take a long time or even be impossible to return to a free-flowing condition without maintenance.
Alarm units operable to alert the user to a low-level condition are also known. Nevertheless, it is desirable to provide a control which not only warns that refilling is needed, but also at least temporarily shuts off the heater or other fuel using apparatus when the oil supply in the storage tank reaches a predetermined low value. It is also desirable to provide a control which is adapted to function with standard tank and indicator designs now in service, and with minimal installation effort. It is further desirable to provide a control with a manual override function such that, if necessary, the heater can still be operated after the low level cutoff, if desired, even until the oil supply is exhausted, and to automatically reset from an override condition when the tank is refilled.